1. Field of the Invention
The present invention relates to a rare earth bonded magnet molded by bonding a rare earth magnet powder with a bonding resin (binder) and a composition of a rare earth bonded magnet to manufacture the same.
2. Description of the Prior Art
A rare earth bonded magnet is manufactured by using a mixture (compound) of a rare earth magnet powder and a binding resin (binder), and molding the mixture under pressure into a desired shape. Commonly employed molding methods include the compaction molding method, the injection molding method and the extrusion method.
The compaction molding method comprises the steps of packing the compound into a pressing die, compacting the packing with pressure to obtain a molded body, and then heating the molded body for setting the thermosetting resin used as the binding resin, thereby manufacturing a magnet. As compared with the other methods, this method permits molding with a smaller amount of binding resin. The resultant magnet contains a smaller amount of resin, and this is advantageous in enhancing magnetic properties. This method suffers, however a low degree of versatility with respect to the magnet shape and a low productivity.
The injection molding method comprises the steps of heating the compound to melt the thermoplastic resin of the compound, injecting the resultant melt into a mold while the melt has a sufficient fluidity, and molding the melt into a prescribed shape of magnet. This method is advantageous in that a high degree of versatility with respect to the shape of magnet is available, permitting easy manufacture of even of irregular shaped magnets. However, because a high level of fluidity of the melt is required during molding, it is necessary to increase the amount of binding resin, leading to a drawback of poor magnetic properties of the resultant magnet.
The extrusion molding method comprises the steps of heating the compound bed into an extruder to melt the thermoplastic resin of the compound, extruding the compound from a mold of the extruder and simultaneously cooling it for solidification, and cutting the resultant long molded body into prescribed lengths, thereby manufacturing a magnet. This molding method has advantages of both the compaction molding method and the injection molding method. More specifically, the extrusion molding method permits freely setting a shape of a magnet through selection of a mold, easy manufacture of a thin-walled or long magnet, and because a high level of melt fluidity is not required, allows molding with a smaller amount of added binding resin than that in the injection molding method, thus contributing to the enhancement of magnetic properties.
As disclosed in Japanese Patent Publication Nos. JP-B-56-31841 and JP-B-56-44561, a thermosetting resin such as an epoxy resin has been used as a binding resin contained in the foregoing compound, and because of the properties of thermosetting resins, it has been possible to use such a small amount of addition as from 0.5 to 4 wt. %.
When employing a thermoplastic resin as the binding resin, however, the effects of the amount of addition and the state of the resin in the bonded magnet on moldability, magnetic properties and mechanical properties have not as yet been clarified.